feat: Agent Registry in ADK

Client library for the Agent Registry API that allows users to discover, look up, and connect to agents and MCP servers cataloged in the registry.

Co-authored-by: Kathy Wu <wukathy@google.com>
PiperOrigin-RevId: 873073675

contributing/samples/agent_registry_agent/README.md

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+# Agent Registry Sample
+
+This sample demonstrates how to use the `AgentRegistry` client to discover agents and MCP servers registered in Google Cloud.
+
+## Setup
+
+1.  Ensure you have Google Cloud credentials configured (e.g., `gcloud auth application-default login`).
+2.  Set the following environment variables:
+
+```bash
+export GOOGLE_CLOUD_PROJECT=your-project-id
+export GOOGLE_CLOUD_LOCATION=global  # or your specific region
+```
+
+3.  Obtain the full resource names for the agents and MCP servers you want to use. You can do this by running the sample script once to list them:
+
+    ```bash
+    python3 agent.py
+    ```
+
+    Alternatively, use `gcloud` to list them:
+
+    ```bash
+    # For agents
+    gcloud alpha agent-registry agents list --project=$GOOGLE_CLOUD_PROJECT --location=$GOOGLE_CLOUD_LOCATION
+
+    # For MCP servers
+    gcloud alpha agent-registry mcp-servers list --project=$GOOGLE_CLOUD_PROJECT --location=$GOOGLE_CLOUD_LOCATION
+    ```
+
+4.  Replace `AGENT_NAME` and `MCP_SERVER_NAME` in `agent.py` with the last part of the resource names (e.g., if the name is `projects/.../agents/my-agent`, use `my-agent`).
+
+## Running the Sample
+
+Run the sample script to list available agents and MCP servers:
+
+```bash
+python3 agent.py
+```
+
+## How it Works
+
+The sample uses `AgentRegistry` to:
+- List registered agents using `list_agents()`.
+- List registered MCP servers using `list_mcp_servers()`.
+
+It also shows (in comments) how to:
+- Get a `RemoteA2aAgent` instance using `get_remote_a2a_agent(name)`.
+- Get an `McpToolset` instance using `get_mcp_toolset(name)`.